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To the mathematically inclined from the mathematically challenged...

I understand the obsession with eliminating measurement errors. But how much error are we really talking about here? Suppose I take a short piece (say 6 inches) of 50 ohm coax cable, crimp a male sma connector on one end, and a couple of alligator clips on the other end. I "calibrate" with the alligator clips dangling (open), then with a short piece of wire (short), and, finally, with a non-wirewound, 50 ohm 1% resistor (load). Then I go to measure inductance of the coil I hand wound for my crystal radio. Are we talking 1% here? 10%? 100%? 1000%?

I understand that the error in such as setup is going to increase with frequency. But, say, between 3 MHz and 30 MHz...? Below that? Above that?

I also understand that there will be many factors affecting this setup: resistance between the alligator clips and whatever you have clipped into them; random capacitance and inductance from nearby stuff; temperature, humidity, spilling your coffee on it... If the effect of all possible factors affecting measurement using such a setup are just too great to make any kind of ball-park assessment of the range of error, you could just say that.

Thanks, ahead of time, for insight into this question.

Hi Jeff;

Per my post:
@ Gary O'Neil - /g/nanovna-users/message/9184

I don't find any source of disagreement in your posts:
@ Jeff Anderson - /g/nanovna-users/message/9178
@ Jeff Anderson - /g/nanovna-users/message/9181

I will also confess that I overstated a Hackborn quote which modified
its more accurate interpretation. He didn't dismiss anything, but
rather makes the statement that all of the errors and uncertainties in
the system are measured and remembered.

By that inexcusable but excellent example of my inability to make and
defend my point; I will attempt instead to understand your
understanding of the process, and search for where the two will
hopefully converge.

After several reads and re-reads of your and Erik's posts; I think you
two may be on the same page. Your post, and another by Dr. Kirby:
@ Dr. Kirby - /g/nanovna-users/message/9183

hint at a possible disconnect in "my" understanding, which may be
linked to a vagueness in the use of jargon, or more pathetically, my
lack of understanding of the jargon in use.

The way I am interpreting your posts, I see the use of the terms
calibration, characterization, and correction. You also identify the
noise and imperfect characterizations of the standards as not being
corrected by the error correction process.... referring to a Hand
quote.

You also make reference to HP and Keysight quotes... both of which I
agree with as being correct. To my point; any statement that the
"accuracy" of something (anything) used for the purpose of improving
the accuracy of the measurement must itself be accurate cannot be
argued. It is made true by the way it is stated and/or presented.

Clearly there is no argument that even with the highest of quality in
the standards, at some upper limit of frequency, the manufacture of
standards sets to the exacting dimensional tolerances required to
guarantee that the reference plane remains constant becomes
unachievable, significant rotational errors occur and corrections for
the known and well defined imperfections are needed in the calibration
in order to make meaningfully accurate measurements.

So my lack of understanding seems to lie in the question being what's
the point of attempting to model imperfect standards of uncertain
accuracy, and using that model to corrupt the ability of the algorithm
to accurately measure and remember all of the system errors and
uncertainties with uncertain guesses at what the ones that are measured
have been characterized to be? Are not the errors that manifest
themselves as problematic, only problematic because they result from
differences in the location of their respective reference planes? the
uncertainties of the parasitic reactance properties associated with
each of the standards are measurable, and thus they will be "measured
and remembered". As such, they are all present and accounted for in the
calibration. Characterization of the standard reference plane location
(degrees per GHz) would seem to be a more precise and accurate manner
to compensate (not calibrate) for their respective rotational offsets
without compromising the integrity of the calibration algorithm. After
that; how precise does the rotational compensation need to be in order
to sufficiently orient the regions of infinity to the VNA user such
they are presented with the most accurate measurement the VNA is
capable of providing?

This brings up yet another question. If the devices are "measured and
remembered" as the math clearly dictates, what would cause a short and
an open (defined as such) to appear anywhere other than the locations
-1 and 1 without biasing the algorithm to place them differently? If
they are intentionally placed at a different location, what is the
justification for doing so, since this would seem to create a need to
compensate for the induced post calibration offset errors ?


--
73

Gary, N3GO

KV5R,

They are the handout for the courses on antenna and feedlines.
As such they are often read this, evaluate this question.

If your going to suck those files up, then get the books that go with them
as many
do reference them for reading. If not you are just wasting disk space.

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Robert,

It would be desirable if you could also measure the transmission through
the
cable, including both magnitude and phase versus frequency. Plus any info
you have about the supposed characteristic impedance and velocity factor of
the cable. Extend the lower frequency limit down to 50 kHz as well.

A second set of equivalent plots but with the nano's scan set for a span
of about
10 MHz and a center frequency well up in the VHF regime (~150 MHz would be
fine) might also be pretty informative. These should also include the
scan of the
termination that you're using by itself.

The overall shape of the plot is somewhat consistent with two point
reflections
of roughly equal magnitude, separated in round-trip time by a bit under
500 nsec.
However, the deteriorating overall return loss at increasing frequency is
not really
consistent with that simple model. But if your connections to the hard
line were
a bit crude, creating added impedance bumps at the two ends, the curve
I've seen
so far would be better-explained.

Could you also send a photo showing exactly how you are physically
interfacing
to the hard line from the small cable environment?

What I've seen so far suggests a cable whose impedance is not really 50
ohms,
combined with some other issue such as (perhaps) impedance bumps at the two
ends. The period of ~2 MHz seen in the S11 plot is not consistent with
a physical
length of 350 ft- in fact it's off by almost a factor of 2 if one assumes
a VF of 0.67.

Dana K8YUM

はぁ (ha), that did the trick. I did the surgery on only one leg so there is still some imbalance, but the directivity improved from 8dB to 16dB, on par with the performance with tuning (see attached). Don't think I'll ever convince TOKO or mini-circuits to build baluns vertically like this though, and it won't fit in the shield can anymore, so I think the tuning will stay for now.